This invention relates to light emitting semiconductor devices, such as lasers and LEDs, and more particularly to the confinement of current flow in these devices.
Nearly two decades ago light emitting semiconductor devices, especially those having a planar p-n junction in a monocrystalline semiconductor body, utilized broad area electrical contacts on opposite major surfaces of the body to apply forward bias voltage and pumping current to the junction. In an LED the resulting radiative recombination of holes and electrons in the active region in the vicinity of the junction generated spontaneous radiation. Primarily, one fundamental modification converted the LED to a laser: a cavity resonator was formed on the semiconductor body by a pair of parallel cleaved crystal facets orthogonal to the junction. When the pumping current exceeded the lasing threshold, the spontaneous radiation, which in the LED was emitted from the active region essentially isotropically, was converted to stimulated radiation, which in the laser was emitted as a collimated beam parallel to the junction and along the resonator axis. Of course, other design considerations played a role in making the advance from LED to laser, but these matters are not discussed here inasmuch as our purpose at this point is merely to state the now well-known kinship between p-n junction lasers and LEDs.
The broad area contacts (e.g., 100 .mu.m wide) on these devices caused the pumping current density at the p-n junction to be relatively low which, therefore, meant that relatively high currents (e.g., hundreds of mA in lasers) were required to achieve desirable radiation power levels. High currents in turn heated the semiconductor body and necessitated coupling the device to a suitable heatsink and/or operation of the device at cryogenic temperatures. The basic solution to this problem was then, and is today, to reduce the area of the p-n junction which has to be pumped so that for a given current density the amount of pumping current required is proportionately lower. One implementation of this solution is to constrain the pumping current to flow in a relatively narrow channel (e.g., 12 .mu.m wide) from a major surface of the semiconductor body through the active region.
One of the earliest structures for constraining current to flow in such a channel was the stripe geometry contact first proposed for semiconductor lasers by R. A. Furnanage and D. K. Wilson (U.S. Pat. No. 3,363,195 issued on Jan. 9, 1968). The stripe geometry reduces the threshold current for lasing (compared to lasers with broad area contacts) and limits the spatial width of the output beam. Since that early proposal, numerous laser configurations have been devised to implement the stripe geometry concept: (1) the oxide stripe laser; (2) the proton bombarded laser; (3)the mesa stripe laser; (4) the reverse-biased p-n junction isolation laser; (5) rib-waveguide lasers; and (6) buried heterostructures of various types.
The most commonly used configuration for the past eleven years, however, has been the proton bombarded, GaAs-AlGaAs double heterostructure (DH) laser described, for example, by H. C. Casey, Jr. and M. B. Panish in Heterostructure Lasers, Part B, pp. 207-210, Academic Press, Inc., N.Y., N.Y. (1978). Despite its various shortcomings, lasers of this type have regularly exhibited projected lifetimes in excess of 100,000 hours and a number have exceeded 1,000,000 hours (based on accelerated aging tests). Long lifetimes have also been projected in DH LEDs employing different contact geometries (e.g., dot-shapes or annular rings) but similar proton bombardment to delineate the current channel.
Several of the shortcomings of proton bombarded DH lasers are discussed by R. W. Dixon et al in The Bell System Technical Journal, Vol. 59, No. 6, pp. 975-985 (1980). They explored experimentally the optical nonlinearity (presence of "kinks" in the light-current (L-I) characteristics) and the threshold current distribution of AlGaAs, proton-bombardment-delineated, stripe geometry DH lasers as a function of stripe width (5, 8, and 12 .mu.m) in cases in which the protons did and did not penetrate the active layer. They demonstrated that shallow proton bombardment with adequately narrow stripes (e.g., 5 .mu.m) can result in satisfactory optical linearity (kinks are driven to non-obtrusive, high current levels) without the threshold penalty that has been associated with narrow-stripe lasers in which the protons penetrate the active layer. On the other hand, lasers with such narrow stripes have exhibited a statistically meaningful, although not demonstrably fundamental, decrease in lifetime. In addition, failure of the protons to penetrate the active layer increases device capacitance and hence reduces speed of response and, moreover, increases lateral current spreading and hence increases spontaneous emission. In digital systems, the latter implies a higher modulation current to achieve a predetermined extinction ratio or a lower extinction ratio for a predetermined modulation current.